1. Technical Field
The present invention relates to a method for producing a thermoplastic synthetic resin sheet or film, by which a thin sheet of a thermoplastic synthetic resin is continuously formed and the surface thereof is made smooth and mirror finished. The present invention also relates to an apparatus for producing the thermoplastic resin sheet or film.
2. Background Art
An example of a known apparatus for producing a thermoplastic resin is disclosed in JP-A-6-170919. This publication describes a method for producing a thermoplastic synthetic resin sheet or film, wherein at the downstream side of an extrusion nozzle of a T-die, a cast drum and a first metal roll located at the upstream side of the cast drum are provided as a set for receiving a thermoplastic synthetic resin extruded from this nozzle into the gap thereof. The peripheral surface of the first metal roll is coated with rubber or an elastomer having an elasticity. At the downstream side of the closest point of the cast drum and the first metal roll, a second metal roll facing the peripheral surface of the cast drum is provided. Between these two metal rolls and a third metal roll, a metal endless belt is entrained with a constant tension. With the coated metal roll, the metal endless belt is pressed against the cast drum. In a section between the first and second metal rolls, the metal endless belt runs along the peripheral surface of the cast drum. Between these two metal rolls, a film-like thermoplastic synthetic resin extruded in a molten state from the T-die is nipped and pressed in an arcuate form with use of the cast drum and the metal endless belt having corrosion resistance, and cooled and formed into a sheet-like shape. Further, as an apparatus for practicing this production method, this publication describes an apparatus for producing a thermoplastic synthetic resin sheet or film, wherein at the downstream side of an extrusion nozzle of a T-die, a cast drum and a first metal roll located at the upstream side of the cast drum, are provided as a set for receiving a thermoplastic synthetic resin material extruded from this nozzle into the gap thereof. The first metal roll is provided so that it moves close to or away from the cast drum. At the downstream side of the closest point of the cast drum and the first metal roll, a second metal roll facing the peripheral surface of the cast drum is provided. Between these two metal rolls and a third metal roll, a metal endless belt is entrained with a constant tension. In a section between the first and second metal rolls, the metal endless belt has a flexibility so that it runs along the peripheral surface of the cast drum. The the surface of the first metal roll for nipping and pressing the metal endless belt against the cast drum is coated with a heat-resistant rubber or an elastomer having an elasticity.
Further, JP-A-7-40370 describes an apparatus for producing a synthetic resin sheet, in which a synthetic resin material continuously supplied and fed from a die is cool-pressed with a casting mechanism having a casting roller to cast a synthetic resin sheet in a predetermined thickness. The casting mechanism of this production apparatus comprises a casting roller rotatably supported on a casting base block; a casting belt means for casting a molten synthetic resin material supplied from the die into a predetermined thickness which circulates in contact with the outer peripheral surface of the casting roller and moves in cooperation with the casting roller; and a pressing allowance-adjusting means for adjusting the pressing force of the surface of the casting belt means against the outer peripheral surface of the casting roller; wherein the casting belt means is constituted by a casting belt entrained around and between a pair of belt rollers supported by a slide frame while adjusting the distances in the upward, downward, frontward and rearward directions, and the casting belt is located so that it faces the outer peripheral surface of the casting roller along the outline of the outer peripheral surface of the casting roller, whereby a molten synthetic resin material fed is pressed to form a thin sheet-like product made of the synthetic resin.
By using the apparatus indicated in the above-mentioned JP-A-6-170919, it becomes possible to produce a glossy thermoplastic synthetic resin sheet or film. However, the spans between the third metal roll and the two metal rolls are long, the metal endless belt entrained between the metal rolls with a constant tension is too long, and spots are formed on the produced thermoplastic synthetic resin sheet or film due to welded joints of the metal endless belt and scars generated by corrosion partially on the metal endless belt, whereby it will be required to replace the metallic belt with new one. For such purpose, however, a few persons (usually from 4 to 6 persons) and substantial time are required to detach the long metal endless belt from the metal roll having a large diameter for replacement, whereby the cost of this production apparatus is increased. In addition, since this apparatus is large in size, a small-size apparatus with low cost is demanded at the present stage. Further, since the metal endless belt is formed by jointing metallic extended thin plates by welding into an endless belt-like form, it is required to process the belt smoothly so that unevenness at the joint will not influence the sheet.
Further, even if the smooth processing is applied, since the welding material and the material of the belt are heterogeneous materials, corrosion is likely to occur at the joint of the metal endless belt, and since the welded portion is slightly different from other portions in the rigidity and thermal strain, its smoothness tends to be different from other portions.
The apparatus disclosed in JP-A-7-40370 does not invite such a trouble, and does not cause inclusion of air at the time of casting, and makes it possible to produce a sheet-like product or the like, of which the surface is uniform to some extent and has a highly mirror-finished property and which has a smooth or appropriately patterned surface. However, only by moving a pair of the belt rollers close to or away from the casting roller by forward or rearward movement of the slide frame, the casting belt is not necessarily located to an appropriate position to face the casting roller, and in addition, it is not easy to accurately and finely adjust the pressing force of respective belt rollers independent of the positional adjustment of the belt rollers.
Further, the synthetic resin material is, by its nature, classified into a synthetic resin material such as acryl which tends to firmly adhere to the casting drum or the casting roller, and a synthetic resin material such as polypropylene which is relatively easily peeled off. However, with conventional apparatuses, the length for nipping and pressing the molten synthetic resin material with the casting belt and the casting roller is substantially constant, and thus, although it depends on the resin material, the time for nipping and pressing is so long that the sheet or film is hardly peeled off from the roller, whereby workability will not be improved, or the time for contacting the roller becomes short, by which adequate gloss can not be obtained.
Under such circumstances, an object of the present invention is to provide a method for producing a thermoplastic synthetic resin sheet or film, and an apparatus for it, by which the production apparatus can be made small in size and easy in handling, and a sheet or film excellent in gloss and smoothness can be obtained.
Another object of the present invention is to provide a method for producing a thermoplastic synthetic resin sheet or film, and an apparatus therefor, in which even if the type of the synthetic resin material is changed, the sheet or film can be easily peeled off from the casting drum or the casting roller, and at the same time, adequate gloss can be obtained.